The present invention is related to methods of determining the beam pattern of an antenna, and more particularly to a method of forming the far-field beam pattern of the antenna from antenna field measurements taken at short ranges by deriving discrete values of the beam pattern for corresponding angle increments along an angular rotation, each discrete value being derived as a function of a multiplicity of antenna field measurements taken about the angle increment corresponding thereto.
FIG. 1 is an illustration of an exemplary site for testing radar antennas. The antenna 10 under test may be coupled to a rotating pedestal 12 which causes rotation of the antenna 10. Both assemblies 10 and 12 may be mounted on a structure 14 which may be a test building, for example, to render the antenna 10 elevated above the ground surface 16 a dimension e1. The structure 14 may be situated at one end 18 of a strip of land 20. At the opposite end 22 of the land strip 20 may be situated a transmitting antenna 24, for example, for transmitting radar signaling having a carrier wavelength .lambda. along a line-of-sight 26 to the antenna 10 under test. The transmitting antenna 24 is located at a finite range R away from the antenna 10 as measured along the line-of-sight 26. The antenna beam pattern of the antenna 10 may be rotated through a rotational angle about the line-of-sight for a collection of antenna field measurements for use in determining the far-field beam pattern of the antenna 10.
Typical apparatus used for the aforementioned determination is shown in the block diagram schematic of FIG. 2. Referring to FIG. 2, the radar signaling received by the antenna system 10 is provided to a conventional radar receiver 30 via rotating pedestal 12 and signal line 32. The receiver 30 conditions the signaling to provide amplitude A and phase .phi. signals to a recording device 34, which may be a strip chart recorder, for example. The recorder 34 may be synchronized to the rotation of the antenna 10 utilizing a sync signal 36. In addition, the amplitude A and phase .phi. signals along with a signal representative of the antenna pedestal position may be digitized in a conventional analog-to-digital (A/D) converter 38. The digitized signals 40 may be provided to a programmed digital computer 42 for processing to determine the far-field beam pattern of the antenna 10.
In general, antennas are designed to operate ideally with planar phase fronts. But, because the antennas are tested with finite ranges R, curved phase fronts are actually produced which result in an error near the main beam. For conventional antennas, the accepted range length requirement is R&gt;2D.sup.2 /.lambda., which produces a maximum phase error of 22.5.degree. across the diameter D of the antenna with a wavelength .lambda.. The effect of the curved phase front, in turn, typically causes a broadening of the base of the main beam. For ultra-low side lobe antennas, the recommended range is 4D.sup.2 /.lambda.. These recommended range sizes may be unavailable and at times undesirable, especially for an ultra-low side lobe antenna, since the larger the range, the more difficult it is to eliminate reflections. For example, a 40-foot L-band ultra-low side lobe antenna calls for a 11/2 mile range with all reflections down 60 dB.
Presently, much attention is being given to testing the antennas at much shorter ranges than those recommended for far-field beam pattern determinations. These shorter ranges are commonly referred to as near-field ranges. One known test method used to convert near-field test measurements into the desired far-field beam pattern of the antenna performs the operation with matrix manipulations. In this method, the antenna 10 is rotated through an angular arc, which may be on the order of 20.degree., for example, and may include as many as 100 angle increments for which antenna field measurements may be taken. The angle increments and antenna field measurements, i.e. amplitude A and phase .phi., are digitized in the A/D converter 38 and provided to the digital computer 42 over signal lines 40. The collection of the antenna field measurements forms the near-field range antenna beam pattern V.sub.R (.theta.).
It has been determined that if the range length is not too short, the angular beam pattern and the aperture distribution across the antenna are naturally related by a Fourier transformation. However, it is necessary to collect all of the antenna field measurement data taken over the angular rotation before the transformation can be performed. According to this method, the digital computer 42 may be programmed to transform the near-field pattern V.sub.R (.theta.) to an effective distribution f(x). Thereafter, the phase curvature effects are removed to obtain the actual distribution g(x). An inverse Fourier transformation is then performed on g(x) to obtain the desired far-field antenna beam pattern V.sub..infin. (.theta.). The aforementioned transformations function well but involve very time consuming and costly computer manipulations. For example, for 100 angular incremental samples, the digital computer 42 must simultaneously solve 100 equations in both .theta. and x which involves processing a 100-by-100 matrix for each of the two transformations.
In view of the above, it would seem completely desirable to avoid the complexity in calculations for determining the far-field antenna beam pattern with shorter-range field measurements for the reason that they are so time consuming, especially on smaller computers. A method which uses various approximations at reasonable ranges in the determination of the far-field antenna beam pattern is proposed to eliminate the onerous transformations presently performed. In addition, a method which results in forming the far-field antenna beam pattern substantially during the taking of the shorter-field measurements would be additionally desirable.